Self-contained mobile extendable tower

ABSTRACT

A self-contained mobile extendable tower for transferring loads, including workmen, between ground and higher levels utilizing a plurality of multiple stage mast assemblies maintained accurately synchronized despite non-uniform load distribution by simple electrical sensor means carried by the load platform. The platform is suspended from yoke means projecting upwardly from the platform and having its upper ends connected to the last stage of the respective mast assemblies. A common prime mover, such as an internal combustion engine or an electric motor, is coupled by a reversible transmission to separate reversible pumps supplying a respective one of the masts with pressurized fluid. The chassis on which the tower is supported is self-propelled and steerable from a control station on the load platform.

limited tates Patent 1191 Galloway SELF-CONTAINED MOBILE EXTENDABLETOWER [75] Inventor: George W. Galloway, Arcadia, Calif.

[73] Assignee: G. W. Galloway Company, Los

Angeles, Calif.

[22] Filed: June 8, 11972 [21] Appl. No.: 261,081

Related [1.8. Application Data [63] Continuation-impart of Ser. No.233,613, March 10,

1972, abandoned.

182/141, 112, 13, 19, 14; 180/63; 254/89 R, 89 H, 93 R, 93 L; 60/97 E;91/171, 189, 412

Primary Examiner-Evon C. Blunk Assistant Examiner-James L. RowlandAttorney, Agent, or FirmSellers and Brace [57] ABSTRACT A self-containedmobile extendable tower for transferring loads, including workmen,between ground and higher levels utilizing a plurality of multiple stagemast assemblies maintained accurately synchronized despite non-uniformload distribution by simple electri- [56] References Cited cal sensormeans carried by the load platform. The platform is suspended from yokemeans projecting up- UNITED STATES PATENTS wardly from the platform andhaving its upper ends 210811248 5/1937 p y 132/141 X connected to thelast stage of the respective mast as 23965 4/1940 Wagner et 182/63 Xsemblies. A common prime mover, such as an internal i g2 combustionengine or an electric motor, is coupled by 289I3S3 6/1959 f fig'g 'g182/13 X a reversible transmission to separate reversible pumps 2 970667 2/1961 Bercaw 8.1:: II: 182/13 x Supplying a respective one of themasts with 3:095:945 7/1963 Mitchel] I I I 182/14 ized fluid. Thechassis on which the tower is supported 3,153,911 10/1964 Mark et.al..... 180/63 X is self-propelled and steerable from a control station3,191,717 6/1965 Hiyama 1. 182/141 X on the load platform. 3,265,3578/1966 Schilling 254/89 H 3,289,868 12/1966 M11161 254/89 11 x 34Claims, 10 Drawmg Flames [7 i m l PATENTEDJA'N 1191s SHEET 10F 5PATENTEI] JAN 7 I975 SHEET 8 OF 5 PATENTEDJA v sum 30F s FIG- 7 PATENTEBT1975 3,858.688

SHEET MP 5 SELF-CONTAINED MGlBILlE EXTENDABLE TOWER This is acontinuation-in-part of application Ser. No. 233,613, filed Mar. 10,1972, now abandoned.

This invention relates to hydraulically extendable towers, and moreparticularly to a self-contained mobile extendable tower apparatusutilizing a plurality of multistage telescopic masts to support the loadplatform and characterized by its overall simplicity, the provision ofhigh precision automatic means for maintaining the platform leveldespite non-uniform distribution of the load, and unidirectional primemover means equipped to drive hydraulic fluid pumps in either directiondepending on the desired direction of load movement.

There have been many proposals heretofore to pro vide load liftingdevices utilizing two or more masts operating in parallel to transferloads between different elevations. These various proposals have beenbeset by many problems including the highly vexatious problems ofconstraining the mast systems to operate in synchronism, particularlyunder unequal load distribution conditions and other variables wellknown to those experienced in the art. These proposals have included theexpedient of using identical mast assemblies supplied with pressurizedfluid by identical pumps driven from a common motor. Under idealconditions such a system provides reasonably satisfactory results.However, a system of this type is highly sensitive to leakage occurringin one or more of the fluid circuits and unavoidably occurring after aperiod of use. Any leakage is greatly aggravated by variations in loaddistribution on the platform. Furthermore, either non-uniform loading orleakage is accumulative during repeated cycling with well known seriousconsequences.

Other proposals for synchronizing and correcting for out of phaseoperation of two or more masts for various reasons utilize synchronizingcontrol expedients of a wide variety including slave cylinder sensingdevices, cable systems interconnecting the upper ends of each mast withservo valve mechanisms, and various level sensing mechanical linkagesconnected to fluid control valves for each mast. These various systemsare subject to many shortcomings including their characteristiccomplexity, the use of many components subject to to]- erancevariations, wear and damage, as well as their high initial andmaintenance costs and the need for skilled technicians to maintain andservice them.

In view of the foregoing and other shortcomings and disadvantagescharacteristic of previously designed hydraulic lifting apparatus, it isa primary object of the present invention to provide a greatly improvedmobile extendable tower apparatus functioning in a simple highlyreliable foolproof manner avoiding the aforementioned and otherdisadvantages of prior constructions. In a typical embodiment, thelifting apparatus utilizes the frame of a low height chassis to rigidlysupport a plurality of multistage masts cooperating to support a workplatform from their upper ends. Preferably, the platform is suspended byyoke means sized to telescopically embrace the larger stage of eachmast. These yokes project upwardly from the opposite ends of theplatform and, in a preferred arrangement, are substantially as long asthe individual stages of the mast thereby permitting the platform to belowered close to ground level when the masts are fully retracted. Theself-propelled chassis includes a complete selfcontained power systemfor the chassis as well as for the hydraulic system. The latter utilizesa single prime mover having a reversible drive to identical positivedisplacement pumps each connected to a respective one of the masts andincluding automatic level control means effective to maintain theplatform level under uniformly distributed loading condition. Sincethese ideal conditions seldom prevail in practice, the invention towerincludes automatic precision synchronizing means for maintaining theplatform level despite unequal load distribution conditions. Thisself-levelling and synchronizing means typically comprises a pair ofsensitive switches responsive to tilting of the platform in eitherdirection from the horizontal to bleed fluid from the highest mast. Asthe platform is restored to horizontal, fluid bleeding is discontinued.The electrical tilt sensor comprises a pair of precision switches whichare connected to simple on-off solenoid bleed valves.

The tower chassis may be propelled forwardly and rearwardly by separatehydraulic motors connected to each of its rear wheels and may be steeredby a hydraulic cylinder connected to the steering control linkage forthe front wheels. The chassis can be propelled and steered at all timesexcept when the tower assembly is being raised or lowered. In the secondembodiment a single motor is connected to a pair of chassis wheels via adifferential unit. In one illustrative embodiment, the two pumps servingthe tower masts are driven by a reversible d.c. motor powered bybatteries carried on the chassis, whereas in a second illustrativeembodiment the pumps are driven by an internal combustion engine or adirect current motor coupled to the pumps by the reversible transmissionmeans. All components for the tower as well as the chassis arecontrollable by a control station located on the platform and suitablyconnected to the chassis by a multiple conductor electrical cable and/orhydraulic fluid lines interconnecting slave motors.

In an alternate control system, the electrical tilt sensor comprises apair of precision switches which are connected to simple on-off solenoidbleed valves located at the base of the tower. This system iselectrically energized through the switch on the platform which isturned on when the reversing control lever of the transmission unit ismoved from the neutral position into either the up or the down position.A second set of tilt sensing switches on the column is used foremergency lowering of the platform when the electric motor or internalcombustion engine is inoperable for any reason. These switches areactivated from the platform upon closing the emergency down switch andthereby open both balancing solenoid valves normally operated as bleedvalves. However, should one mast lower faster than the other thesolenoid valve for that column closes until the platform is level andthen opens.

It is therefore a primary object of the invention to provide an improvedrugged and highly reliable selfpropelled self-contained mobile towerassembly.

Another object of the invention is the provision of a high capacityextendable mast assembly utilizing a plurality of multistage masts andhaving improved simplified means for maintaining the load platform leveldespite non-uniform load distribution.

Another object of the invention is the provision of an extendable towerapparatus utilizing a pair of multistage supporting masts havingimproved means for maintaining the platform level and wherein theplatform proper is suspended from yoke means projecting upwardly fromthe opposite ends of the platform.

Another object of the invention is the provision of a self-poweredlifting apparatus utilizing a plurality of multistage masts powered byunidirectional prime mover means coupled to a separate fluid pump foreach mast through reversible transmission means.

Another object of the invention is the provision of lifting apparatusemploying a plurality of multistage masts arranged in parallel andpowered by unidirectional prime mover means for supplying or releasingequal quantities fo fluid to and from the masts, depending on thedirection of mast movement, via separate positive displacement pumpsselectively driven in either direction by reversible transmission meanscoupling said pumps to the prime mover means.

Another object of the invention is the provision of a mobile largecapacity multi-mast load lifting apparatus powered entirely from anon-board internal combustion engine driving separate synchronized pumpsfor each mast through a single reversible power transmission unit andoperable to lock the load at any selected level by placing the powertransmission unit in neutral.

Another object of the invention is the provision of a multi-mast liftingapparatus equipped with emergency control means for lowering the loadplatform by simultaneously bleeding fluid from both masts throughidentical bleed ports and including means for automatically closingeither bleed port so long as the associated mast is lower than the otherthereby to maintain the load platform substantially level duringemergency lowering conditions.

Another object of the invention is the provision of lifting apparatusutilizing a plurality of telescopic masts and electrical sensor meansfor detecting and compensating for non-uniform operation of the masts.

Another object of the invention is the provision of an extendable towersupported by a plurality of multistage masts movable in either directionby pressurized fluid flowing through reversible positive displacementpumps coupled to reversible drive means and driven by a common primemover.

Another object of the invention is the provision of an automatic brakingsystem for the tower chassis and a smooth acceleration and decelerationdrive to the tower chassis.

These and other more specific objects will appear upon reading thefollowing specification and claims and upon considering in connectiontherewith the attached drawing to which they relate.

Referring now to the drawing in which a preferred embodiment of theinvention is illustrated:

FIG. 1 is a top plan view of one illustrative embodiment of theinvention mobile lifting apparatus;

FIG. 2 is a side elevational view of FIG. 1 showing the platformpartially extended in dot and dash lines and fully retracted in fulllines;

FIG. 3 is a fragmentary view, partially in section, of the steeringmechanism showing the tongue in full lines in its lowered haulingposition and locked in an elevated non-hauling position in dot and dashlines;

FIG. 4 is a top plan view on an enlarged scale of the platform tiltsensor device;

FIG. 5 is a fragmentary vertical sectional view taken along line 5-5 onFIG. 4;

FIG. 6 is a view similar to FIG. 5 but taken along line 66 on FIG. 5;

FIG. 7 is a schematic view of the hydraulic system and of the controlsfor the chassis as well as for operating the tower assembly;

FIG. 8 is a top plan view of a second illustrative embodiment of theinvention;

FIG. 9 is a side elevational view of FIG. 8 corresponding to the showingin FIG. 2; and

FIG. 10 is a schematic view of the hydraulic system and of the controlsfor the chassis and tower assembly of the second embodiment.

FIRST EMBODIMENT Referring initially and more particularly to FIGS. 1and 2, a first embodiment of the invention lifting apparatus, designatedgenerally 10, is shown rigidly supported on a chassis 11 having rearwheels 12, steerable front wheels 13 and a draft tongue 14. The rigidmain frame 15 of the chassis serves additionally as the tower base frameto which there is rigidly secured a pair of identical multistagehydraulic mast cylinders 17,18. The larger diameter lower stages 19,20of this mast assembly are rigidly secured to chassis frame 15 bysuitable means including bracing brackets 21.

The load carrying platform 22 is constructed of structural members andpreferably includes a suitable guard railing 23. Platform 22 ispreferably suspended from the upper ends of the smallest diameter stagesof the mast by suitable yoke means. As herein shown, this yoke comprisesa pair of similar tubes 26,27 having their closed upper ends secured tothe upper end of mast stages 24,25 and their lower ends rigidly anchoredto platform 22 as by base plates 28 (FIG. 1).

Referring more particularly to FIG. 1, it is pointed out that chassis 11may be towed by the hauling tongue 14 which is pivotally connected tothe forward end of the chassis frame by pin 31. Steering links 32interconnect the tongue to steering knuckles 33 fixed to pivotable stubaxles for front wheels 13. When the chassis is not being towed tongue 14may be pivoted upwardly about hinge pin 34 (FIG. 3) and lashed in thisretracted position by latch pin 36 having its free end seatable in notch37. A double action hydraulic cylinder 40 is employed to steer frontwheels 13, it being observed from FIG. 1 that one end of this cylinderis pivotally connected to the chassis frame at 41 and that the free endof its piston rod is connected to the tongue at 42.

The forward end of tongue 14 may include an automatic brake controldevice 45 for the brakes of rear wheels 12. Device 45 includes a chambercharged with brake fluid 46 in communication through a flexible hose 47with the wheel brake cylinders. So long as the tower is being hauledalong the highway, piston 48 is held in its forward position wherein thebrakes are released. However, if the hauling vehicle is braked thenpiston 48 shifts rearwardly under inertia forces and pressurizes fluid46 to apply the brakes of the rear chassis wheels 12.

When the tower is not in a travel mode the brakes may be set by handcontrol lever 50 mounted on the side of the chassis frame and connectedto the wheel brakes by an operating linkage 51 of any suitableconstruction. While in use, the tower assembly is firmly stabilized byhand operated out rigger screws 53 (FIGS. 1 and 2). The threaded shanksof these screws are mounted in threaded bores of brackets 54- and areoperated by hand cranks 55.

Rear wheels 12, as herein shown, are driven by reversible hydraulicmotors 58 rigidly secured to the chassis frame and connected to the rearwheels by a suitable normally engaged but releasable clutch 59 and achain drive 60.

Referring now to FIG. 7, there will be described the hydraulic systemand the controls therefor. As there shown the power supply comprises sixheavy duty storage batteries 62. These batteries provide all powerrequirements including those of the reversible prime mover, such asmotor 65, coupled through the belt drive 66 to a common shaft 67 drivingthe identical positive displacement pumps 68,69. The power supply toprime mover motor 65 is controlled by a pair of switches 70,86 normallypositioned as shown to complete a power supply circuit to the chassisdrive motors 59,59 and to steering motor 40 provided the key operatedswitch 105 is closed. Pumps 68 and 69 are connected to the hydraulicfluid reservoir 75 by conduit 76 leading into each pump and includes afilter 77 and a normally closed check valve '78. The other side of eachof the pumps 68,69 is connected to a respective one of the mastassemblies 17,18 by conduits 79,811. Each of these conduits includes anormally closed pressure relief valve 81 discharging back to the fluidreservoir in conventional manner in the event the pressure in the mastexceeds a predetermined safe value. A similar pressure relief valve 82is shown connected to the supply side of pumps 68,69 and opens if thepressure in line 76 becomes excessive during retraction of the platform.

During lowering of the platform pumps 68,69 are operated in the reversedirection by the reversible prime mover 65, it being understood that thenormally closed pilot operated solenoid valve 85 is first energized byclosing switch 86 to energize relay 65b so that the fluid dischargingfrom pumps 68,69 then passes through conduits 76,87, valve 85 and thencealong conduit 88 back to reservoir 75.

The electrical level sensor, designated generally 90, for maintainingplatform 22 level even though the load is distributed in a grosslynon-unifrom manner, will now be described with reference to FIGS. 1 7.As there shown, the top of the last stage 24 of the multistage mast 17is provided with a cap plate 91 supporting a pair of upright studs 92,92projecting loosely through openings in a cap plate 93 for the tubularyoke 26. Keeper nuts 941 are threaded to the upper ends of studs 92 butdo not bear against plate 93. A pair of fulcrum cylinders 96 are fixedto plate 91 with their upper side bearing against the underside of theyoke plate 93. Keeper bars 94 (FIG. 5) are welded to plate 93 alongeither side of the fulcrum cylinders 96 and cooperate therewith inholding the parts properly centered while leaving the platform and yoketube 26 free to pivot about the upper edge of cylinders 96, when it willbe understood extend crosswise of one end of the platform 22. Thedescribed pivoting connection between the top mast 17 and one end of theplatform, though beneficial, is not essential. Thus, satisfactoryresults are achieved if fulcrum cylinders 96 are replaced with a lowheight strip of metal having its upper and lower edges welded to plates91 and 93.

The electrical sensor proper comprises a pair of normally open microswitches 199a and 1110b rigidly secured to plate 93 by brackets 101. Theactuating buttons or plungers 102 of each of the switches projectthrough openings in plate 93 and bear lightly against the broad head ofrespective adjustable screws 103 carried by plate 91 of mast stage 24.So long as the platform is level both switches a and 10019 will be openas shown in FIG. 7.

The two level sensing switches 100a,1lll)b are connected in circuit withrespective actuating solenoids of normally closed pilot operated fluidventing valves and 111. The inlet sides of these valves are connected tomast 17 and 18 and their discharge sides lead back to fluid reservoir75. Should the one or the other of the masts 17,18 be higher than theothers, it will be understood that the associated one of themicroswitches 190a or 1611b will close and energize one of the ventingvalves 110 or 111 and release fluid from that mast until the platform islevel at which time the closed sensing switches will open andde-energize the activated venting valve.

The hydraulic system for the described tower sup porting chassisincludes a motor 116 driving a pump 117 whenever one or both of switchesand 128 are closed to propel and/or steer the chassis. Pump 117 thenprovides pressurized fluid from tank 75 which flows through thedistributing line 118 to the four way tandem-center valves 119 and 1211.If these valves are deactivated then their respective spools are in aneutral position and the pressurized fluid supply by pump 117 simplyflows through each valve and thence back to the reservoir.

Four way valve 119 controls the flow of fluid to the hydraulic motors59,59 employed to propel the tower via the rear carriage wheels 12,12,the direction of propulsion depending upon whether valve 119 isenergized to the right or to the left. Since it is highly undesirable tostart or stop the chassis abruptly, each supply line leading to motors59 preferably includes an accumulator 122,123 operating in known mannerto absorb shocks incident to abrupt operation of valve 119 therebyproviding smooth acceleration and deceleration of the chassis.Propulsion of the chassis is controlled by switch 125 normally biased toits open position. If this is pivoted forwardly, the chassis ispropelled in this direction, whereas it is propelled rearwardly if theswitch is pivoted rearwardly.

Valve 126 controls the steering cylinder 40 for the front carriagewheels 13,13. Normally this valve is in its neutral position but if thenormally open switch 128 closed to the right, the front wheels steer tothe right whereas if it is closed to the left these wheels steer to theleft. A normally closed pressure relief valve 130 is connected to line118 and opens to relieve pressure in that line if for any reason thefluid pressure becomes excessive.

OPERATION OF FIRST EMBODIMENT The operation of the described mastassembly will be quite apparent from the foregoing description of itscomponents. Let it be assumed that a load is in position on platfonn 22for transfer from near ground level to an elevated height such as 30feet. The operator, standing at control station 1 16 at the forward endof the tower, first operates the appropriate control buttons to movetower 10 to a desired position. For example, the key operated switch 105is closed to supply power to the up and down control switches 76,86,respectively, as well as to the chassis power control switches 125,128.

Assuming the operator wishes to move the tower to a particular plane foruse he pivots the operating handle of toggle switch 125 forward therebyenergizing the power control relay 116a or motor 116 of pump 117 and theforward coil of four-way valve 119 to supply pressurized fluid to thedrive motors 59. While holding switch 125 closed in this forwardposition, the operator may wish to steer the chassis to the right or theleft. This is accomplished by pivoting switch 128 to the right or leftagain energizing motor control relay 116a and the respective ones of thecontrol coils of four-way valve 120 to supply fluid to the steeringmotor 40. Either one or both of switches 125 and 128 are held closed ineither direction so long as the operator wishes to move and to steer thechassis. As soon as he releases his grip of the respective controlhandles, either or both switches move to their normal or open position.

Once the tower is in position the hand brake lever 50 is set and thestabilizing strut devices 53 are lowered firmly against the ground, oragainst levelling blocks if it is necessary to use such blocks to assurehaving platform 22 in a level position as it must be before raising itto an elevated position.

Raising a load present on platform 22 is accomplished by closing the upswitch 70 and holding it closed against the pressure of the springbiasing the switch to its normal position illustrated in FIG. 7. So longas the switch is held depressed, its lower pair of contacts are heldopen thereby deactivating the power supply via conductor 135 to thechassis power units controlled by switches 125 and 128. Accordingly itis impossible for the operator to move the chassis so long as he isendeavoring to operate the mast assembly in either direction. Closingswitch 70 supplies power via conductor 136 to the platform level sensor90 as well as to the coil of the up power relay 65a. Energization ofrelay 65a closes the upper pair of contacts thereby supplying power vialead 138 to drive motor 65 and pumps 68 and 69 in a direction to elevatethe platform. Equal volumes of pressurized fluids are then withdrawnfrom tank 75 by these pumps, the fluid flowing upwardly through filter77, past check valve 78, through each of the pumps and via conduits79,80 into the lower ends of each of the masts 17,18.

If the load is unevenly distributed on the platform, it is likely thatthe less heavily loaded mast will extend faster than the other one.Should this occur, the resulting tilting of the platform from ahorizontal plane is immediately sensed by one of the levelling switches100a or 100b. If mast 17 is less heavily loaded, then switch 100a willclose briefly, thereby energizing venting valve 110 to release a portionof the fluid flowing to mast l7 and allowing such vented fluid to returnto the reservoir. Desirably only a small portion of the fluid flowing tothe higher mast is released so that the platform continues to risesubstantially at the same rate during the level compensating operation.As the platform approaches or reaches a level condition, switch 100aopens so that valve 110 immediately closes.

Should valve 110 overcompensate, then sensing switch closes to openvalve 111 to vent fluid from mast 18. It will therefore be evident thatthe platform continues to rise and that any uneven extension of eithermast is immediately sensed and corrected automatically and withoutattention from the operator.

Once the load is at a desired elevation, switch is opened by theoperator and check valve 78 blocks all return flow thereby holding theplatform rigidly and firmly in its elevated position.

To lower the platform, the operator holds the down" switch 86 closedthereby again discontinuing the power supply to the chassis propulsioncomponents by opening the lower pair of contacts of this switch. Closingthe upper pair of contacts of switch 86 resumes the supply of power tolevel sensor via conductor 136 and also supplies power to the down"control relay 65b thereby reversing the polarity of the power supply tomotor 65 so that this motor now operates pump 68,69 in the reversedirection to pump fluid out of masts 17,18. At the same time powersupply through the middle pair of contacts of switch 86 energizessolenoid valve 85 to open this valve to by-pass the fluid from each ofthe masts around check valve 78.

The energizing power supplied to relay 65b will also be observed asflowing through the normally closed safety switch 141. This switch ismounted on the chassis in the path of the platform and is spring biasedto its closed position. However, if the operator holds the down controlswitch 86 closed for too long a period of time the platform will strikethe control button of switch 141 and open the switch therebydiscontinuing the supply of power to both pump motor 65 as well as tothe normally closed valve 85. It will be recalled that this valve ifopen permits the fluid discharging from the mast by way of pump 68,69 toflow downwardly through conduits 76, through the now open valve 85 andback to the reservoir via conduit 88 and filter 77.

If during lowering of the platform either mast retracts faster than theother the resulting non-level condition of the platform will againoperate level control switches a and 10011 as necessary to bleed fluidfrom the higher one of the masts via valves or 111 until the platform 22is restored to its level condition.

As will be apparent from the foregoing, the platform can be stopped atany level whereupon it is automatically locked in this position by thecheck valve 78. So long as the platform is stationary the power supplyto the level sensor 90 is deactivated. Accordingly, the operating crewmay move freely about the platform and may shift the load onto or offthe platform without any possibility of the platform shifting becausecheck valve 78 prevents the discharge of fluid from either mast andpumps 68,69 cooperate in preventing the transfer of fluid from one mastto the other.

SECOND EMBODIMENT Referring now to FIGS. 8-10 there is shown a secondpreferred embodiment of the load lifting apparatus and differing only inminor respects from the first described embodiment. Accordingly, thesame or similar parts are designated by the same reference characters asin the first described embodiment but distinguished by the addition of aprime. A basic difference resides in the fact that the prime mover 65comprises a unidirectional power unit such as an internal combustionengine or a direct current motor coupled to the identical hydraulic pump68',69' by a reversible hydrostatic transmission of any ssuitable type.A particularly suitable reversible transmission unit sold under thetradename Marshallmatic Model 10 manufactured by the Eaton- MarshallDivision of Eaton, Yale & Towne,lnc., Marshall, Michigan. The use of aunidirectional prime mover in combination with a reversible transmissioneliminates the need for reversing switch control mechanism on theplatform and permits the use of simple slave cylinder control in lieuthereof. Other changes to be described in greater detail include theprovision of a belt takeoff from the prime mover to the hydraulic pumpsupplying fluid to the chassis power units as well as an improvedemergency control accessory and a variation of the automatic levelcontrol mechanism. A typ ical operating arrangement utilizing aninternal combustion engine as the prime mover will now be described.

It will be understood that the gasoline powered engine 65 is coupleddirectly to the reversible transmission unit 145 having its outputcoupled directly to a common shaft driving pumps 68,69. A power takeoffbelt 66 located between the engine and transmission drives pump 117' andprovides a supply of pressurized fluid for the power driving andsteering components of the chassis so long as the prime mover isoperating.

Transmission unit 145 is provided with a reversing control lever 146shown in its neutral position and having an operating connection to thepiston of a slave cylinder 147. The latter is connected in a closedfluid circuit 148 with a control cylinder 149 connected to a controllever 150. When lever 150 is pivoted to the right, lever 146 on thetransmission is likewise pivoted to the right and transmission unit 145is then operative to drive pumps 68',69 in a direction to supply equalquantities of pressurized fluid to masts 17',18 to elevate the loadplatform 22'.

Likewise if control lever 150 on the platform is shifted to the left asviewed in FIG. the transmission control lever 146 is pivoted in the samedirection to its alternate position reversing the direction in whichpumps 68 and 69' are driven. It will be understood that when controllevers 146 and 150 are in neutral position an automatic check valve inthe transmission unit blocks the circulation of fluid within thetransmission means with the result that the transmission remainsstationary and the platform is held firmly at any elevation occupiedwhen the transmission was placed in neutral. This condition is alsoassured by check valve 78' in the fluid supply line leading to theinlets of pumps 68' and 69. The transmission also preferably includes anaccumulator 152 connected to its internal fluid circuit and effective toabsorb shockloads accompanying operation of control lever 146.

Referring to FIG. 9, it will be understood that the flexible hoses 148interconnecting cylinders 147 and 1419, as well as all electric leadsextending between the platform and the chassis are preferably enclosedin two or more protective housings or ducts 166 arranged end to end andmovably interconnected by hinge means 167. Likewise one of their remoteends is hinged at 168 to the chassis and the other is hinged at 169 tothe platform. It will therefore be appreciated that these hoses andelectrical leads are fully protected at all times and that foolprooftrouble free means is provided for extending and retracting the same asthe platform rises and descends.

Engine 65 is provided with a starter motor 155 connected in circuit witha starter relay 156 which may be energized by a normally open manualswitch 157 on the platform or a similar switch 158 at ground level.Likewise a magneto 159 for the engine may be shortcircuited to stop theengine from either stop switch 160 at ground level or a second switch161 located at the control station on the platform.

The automatic self-levelling control for the platform is generallysimilar to that described for the first embodiment but differs in theprovision of an extra pair of switches 1000 and d operatively associatedrespectively with microswitch 100a and 10%. When the platform is in alevel condition switches 100a and 100b are open, whereas switches 100Cand 100d are closed. Normally, switches l00c,100d remain inactivebecause the emergency switch 190 is open. The purpose of the emergencyswitch 190 is to permit the operator to lower the platform in anemergency even though the prime mover is disabled or inoperative for anyreason.

OPERATION OF SECOND EMBODIMENT In use, the second embodiment operates asfollows. The operator first starts engine 65' by closing either of thestarter switches 157 or 158 thereby closing relay 156 and supplyingpower to the starter motor 155 for the engine. As soon as the enginestarts power is delivered via belt 66 to drive pump 117 and provide asupply of pressurized fluid from the supply reservoir 75 for the chassispower units. The lifting apparatus may now be moved to a place of use byoperating the control switch 125 to either the forward or reverseposition from its normal open position thereby diverting pressurizedfluid to drive the single propelling motor 59' forwardly or rearwardlydepending upon the direction of closure of switch 125'.

Referring to FIG. 8 it will be observed that the propelling motor 58'has a power connection to a drive shaft 164 equipped with a differential165. One side of the differential is connected by a chain drive 60' todrive one of the chassis wheels whereas the other side of thedifferential drives a similar chain 60' connected with the otherpropelling wheel. If the operator wishes to steer the front wheels heoperates control switch 128' to the left or the right to operate thefour-way control valve supplying pressurized fluid to the opposite endsof steering cylinder 40' in the same manner as described above inconnection with the first embodiment. If both switches and 128' are inneutral position, as they normally are, then the output of pump 117merely circulates through the respective four-way valve l19,120' andreturns to the reservoir so long as these valves are in neutralposition. Any excess fluid pressure developed by the pump is relieved bythe pressure relief valve Once the apparatus is in the desired positionof use, the stabilizing struts 53' (FIG. 9) are lowered from the chassisframe and a check is made to determine that the load platform is level.Platform 22' may now be raised from a control station on the platform byshifting control lever 150 to the right, as viewed in FIGS. 8 and 10,thereby closing switch 70' and simultaneously shifting the piston ofcylinder 149 to the left which is effective to shift the piston of slavecylinder 147 to the right. The closing of switch 70 arms the contacts ofswitch 100a, 1011b of the platform level control device 90. Likewise themovement of the transmission control lever M6 to the right shiftstransmission to drive pumps 68' and 69' in a direction to supply equalquantities of pressurized fluid to each of the masts l7,18'. Theplatform together with its load is now elevated.

Should either end of the platform start to rise at a faster rate thanthe other end, the tilt so produced acts to close one or the other ofthe level control switches 100a,100b thereby energizing the solenoid ofone of the bleed valves ll',lll to bleed a portion of the fluid beingsupplied to the .higher mast until the level condition of the platformis restored. As this occurs, the closed one of the level switches openswith the result that the open bleed valve recloses.

Once the platform is in elevated position the operator restores thecontrol lever 150 to its neutral position and thereby shifting thetransmission 145 to its neutral position. Fluid cannot return to thereservoir from either mast because this is positively blocked by checkvalve 78' in the supply line to the two pumps.

If the operator wishes to shift the work platform to a lower level hemerely operates control lever 150 to the left as viewed in FIG. 10closing switch 86' to re-arm the level control switches 100a',100b'. Atthe same time the transmission control lever 146 is shifted to the leftto operate pumps 68,69 in the reverse direction. These pumps then pumpfluid from each of the masts and return it to the reservoir via conduit87', bleed valve 85 and conduit 88, valve 85 remaining in its openposition so long as switch 86 is closed. Should either end of theplatform lag behind the other during the descent switches l00a,l00b'operate in the same manner as described above to bleed additional fluidfrom the higher mast until the level condition is restored.

Let it now be assumed that an emergency exists by virtue of the failureof the prime mover while the platform is in a raised position. In thisevent, workmen on the platform can lower the platform without need ofthe prime mover. An operator merely closes the normally open emergencyswitch 190 thereby supplying power to the normally closed pair ofmicroswitches 1000,100d of the level control mechanism 90. Immediatelythat switch 190 closes power is supplied to each of the solenoid bleedvalves l10',l11 thereby opening these valves and allowing equalquantities of fluid to bleed from each of the masts and return to thereservoir 75. Should either end of the platform start to lag the higherend will operate to open the associated one of switches 100e, 100dthereby de-energizing the solenoid of the relief valve for the mastunder the lower end of the platform thereby closing that valve andallowing the higher end of the platform to approach a level conditionwhereupon valve 110' reopens. It will therefore be understood thatswitches 100c,l00d operate in the reverse manner of switches 100a',100b.

While the particular self-contained mobile extendable tower herein shownand disclosed in detail is fully capable of attaining the objects andproviding the advantages hereinbefore stated, it is to be understoodthat it is merely illustrative of the presently preferred embodiment ofthe invention and that no limitations are intended to the detail ofconstruction or design herein shown other than as defined in theappended claims.

I claim:

1. Apparatus for moving a load between different levels comprising incombination: a horizontal load platform, a main frame underlying saidplatform, a plurality of multistage mast assemblies arranged in paralleland interconnecting said main frame and the opposite ends of saidplatform and including an elongated horizontal axis pivot connectionextending transversely of and interconnecting one end of said platformand one of said mast assemblies, separate hydraulic pumps of identicaldisplacement connected in circuit with a respective one of said mastassemblies to supply pressurized fluid thereto and operable in eitherdirection by common prime mover means to extend and retract said mastassemblies in unison, normally closed electrically actuated bleed valvemeans for bleeding fluid from the higher one of said ram assemblies asnecessary to maintain said platform level despite the non-uniformloading of said mast assemblies during both extension and retraction ofsaid platform, and non-level sensor means connected to said platform andresponsive to pivotal movement of said platform about the axis of saidpivotal connection to operate a selected one of said bleed valve meansto bleed fluid from any higher one of said mast assemblies as necessaryto maintain said platform level.

2. Apparatus as defined in claim 1 characterized in that said non levelsensor means is closely adjacent the upper end of one of said mastassemblies.

3. Apparatus as defined in claim 1 characterized in that said non-levelsensor means is mounted on the upper end portion of one of said mastassemblies.

4. Apparatus as defined in claim 1 characterized in that said sensormeans includes an electric switch connected in circuit with electricaloperating means for said bleed valve means and normally so positionedthat said bleed valve means is closed to block the escape of fluid fromall of said mast assemblies so long as said platform is level andoperable to open the bleed valve means of the highest one of said mastassemblies when said platform is not substantially level.

5. Apparatus as defined in claim 1 characterized in that said main frameis rigidly secured to a chassis having steerable front wheels andhydraulically power driven rear wheels, and control means for said powerdriven rear wheels and including accumulator means for cushioninginitiation and cutoff of pressurized fluid to said power drive therebyto assure smooth starting and stopping of said chassis.

6. Apparatus as defined in claim 1 characterized in that said primemover means comprises a reversible electric motor.

7. Apparatus as defined in claim 6 characterized in that said primemover means comprises a reversible direct current motor energized frombatteries carried by said main frame.

8. Apparatus as defined in claim 1 characterized in the provision ofhydraulically powered chassis means supporting said main frame, andmeans for selectively operating at any time either said mast assembliesto change the level thereof or said hydraulically powered chassis means.

9. Apparatus as defined in claim 8 characterized in that saidhydraulically powered chassis means includes accumulator means incircuit with the pressurized hydraulic fluid powering said chassis meanseffective to provide smooth starting and stopping thereof.

10. Apparatus as defined in claim 1 characterized in the provision offolding protective means hingedly interconnecting said platform and saidmain frame and protectively enclosing portions of the controls for saidprime mover and said pumps.

1 1. Apparatus as defined in claim 10 characterized in that said foldingprotective means comprises a plurality of elongated rigid membershingedly connected together in end to end relation with one end movablysupported at said platform and the other end movably supported at saidmain frame, and flexible control means protected and supported by saidrigid members and interconnecting control means for said apparatuslocated on said platform and other portions of said apparatus located onsaid main frame.

12. Apparatus as defined in claim 1 characterized in that said primemover means comprises unidirectional power generating means coupled tosaid separate hydraulic pumps by reversible power transmission means.

13. Apparatus as defined in claim 12 characterized in that said mainframe comprises a hydraulically powered steerable chassis, and fluidpump means having a driving connection with said unidirectional powergenerating means in advance of the drive connection of the latter tosaid reversible power transmission means.

14. Apparatus as defined in claim 12 characterized in the provision ofcontrol means on said platform operable to stop and start said powergenerating means at will from an operating control station on saidplatform irrespective of the elevation thereof above ground level.

15. Apparatus as defined in claim 11 characterized in that said primemover means comprises an internal combustion engine coupled to saidseparate hydraulic pumps by reversible power transmission means.

16. Apparatus as defined in claim 15 characterized in the provision ofcontrol means for shifting said power transmission means between aneutral position, forward drive position and reverse drive position froma control station located on said platform.

17. Apparatus as defined in claim 16 characterized in that said controlmeans includes hydraulic motor means located in part on said platformand in part on said main frame.

18. Apparatus as defined in claim ll characterized in that said mainframe is mounted on steerable front wheels and hydraulically driven rearwheels.

19. Apparatus as defined in claim 18 characterized in the provision ofhydraulically powered means for steering said front wheels from acontrol station on said platform.

20. A tower assembly as defined in claim 19 characterized in theprovision of means for deactivating the operation of said mastassemblies so long as power is being supplied to propel and/or to steersaid chassis.

2B. A tower assembly as defined in claim 19 characterized in theprovision of means for automatically rendering the drive means for thepumps connected in circuit with said mast assemblies ineffective todrive said pumps while said hydraulically powered means for said frontand rear wheels is activated.

22. A mobile chassis-supported extendable tower assembly for moving aload between different levels, said tower assembly comprising a mainchassis frame closely spaced above the ground, a load platform overlyingsaid main frame including a plurality of inverted upright yoke meansoverlying said platform and having the lower ends thereof fixed to saidplatform near the opposite ends thereof, multistage mast assemblieshaving their lower ends rigidly secured to said main frame and theirupper ends secured to the upper end of a respective one of said yokemeans, said mast assemblies having a retracted height correspondinggenerally to the height of said yoke means, power-driven pressurizedfluid means for extending and retracting said masts in synchronism bypressurized fluid supplied to each of said mast assemblies, said lastmentioned means including electrical level sensor means mounted on saidplatform effective to sense tilting of said platform away from thenormal horizontal plane thereof and electrically connected to normallyclosed fluid bleed valves for each of said mast assemblies supportedindependently of said platform and automatically operable to bleed fluidfrom the higher one of said mast assemblies when and as necessary torestore said platform to the level condition thereof during bothextension and retraction of said platform, said assembly being fullyselfcontained and operable from a source of power carried by saidchassis.

23. A tower assembly as defined in claim 22 characterized in that saidelectrical sensor means comprises a pair of electrical switches spacedfrom one another in a plane parallel to a plane common to the axes ofsaid mast assemblies and respectively responsive to a clockwise andcounterclockwise tilting of said platform away from the normalhorizontal position thereof to open one or the other of an associatedone of said fluid bleed valves until said platform has been restored tothe level condition thereof.

24. A tower assembly as defined in claim 22 characterized in theprovision of check valve means located in the fluid supply to said mastassemblies and effective to hold said platform locked in any elevatedposition thereof despite a failure in the operating condition of saidpower driven means, and manually actuated means operable to bleed fluidfrom each of said mast assemblies to lower the same in synchronism.

25. A tower assembly as defined in claim 22 characterized in that saidpower driven means includes similar reversible positive displacementpumps for each of said mast assemblies coupled to a common prime moverfor operation in unison, check valve means between a source of hydraulicfluid and the inlet to said pump means to prevent reverse flow of fluidto the fluid source upon the unintended failure of said prime mover, andmanually actuated fluid venting means between said check valve means andsaid pump means operable to lower said platform as the fluid in saidmast assemblies escapes therefrom via said pump means and thence throughsaid fluid venting means.

26. A tower assembly as defined in claim 22 characterized in theprovision of manually controlled hydraulic motor means for driving anaxially aligned pair of said chassis wheels and including accumulatormeans for cushioning acceleration and deceleration of said chassis.

27. A tower assembly as defined in claim 22 characterized in theprovision of separate reversible hydraulic motors driving a respectiveone of the chassis wheels to propel said mobile tower assembly forwardlyand rearwardly at the operators option.

28. A tower assembly as defined in claim 27 characterized in theprovision of reversible hydraulic means for steering said tower assemblyto the right and to the left from a control station on said platform.

29. A mobile extendable tower assembly comprising a heavy duty chassishaving an elongated rigid main frame, an elongated load supportingplatform overlying said main frame, similar upright tubular housingshaving their lower ends fixed to the opposite ends of said platform, apair of multi-stage mast assemblies having their smaller ends secured tothe interior upper ends of said housings and extending downwardlytherethrough to fixed anchorages at the opposite ends of said chassismain frame, a prime mover coupled to separate reversible pump meansoperable to supply pressurized fluid to a respective one of said mastassemblies to raise and lower the same in unison, normally closedbleeder valve means supported independently of said platform for ventingfluid from any higher one of said mast assemblies if said platform tiltsaway from a horizontal plane lengthwise thereof thereby to restore thelevel condition of said platform, and high sensitivity nonlevel sensormeans including two pairs of electric contactors connected in circuitwith a source of electrical energy and said bleeder valve means andoperable to bleed fluid from any higher one of said mast assembliesduring both extension and retraction of said platform as necessary torestore said platform to a level condition upon sensing a non-levelcondition of said platform.

30. A tower assembly as defined in claim 29 characterized in that saidprime mover is coupled to said reversible pump means by reversibletransmission means.

31. A tower assembly as defined in claim 29 characterized in theprovision of means for lowering said platform independently of theoperation of said prime mover and under the control of said non-levelsensor means.

32. A tower assembly as defined in claim 29 charac terized in that saidpairs of contactors are mounted at the upper end of one of said tubularhousings.

33. A tower assembly as defined in claim 32 characterized in that saidprime mover means comprises an internal combustion engine.

34. A tower assembly as defined in claim 33 characterized in theprovision of manually operable control means on said platform forstarting and stopping said engine.

1. Apparatus for moving a load between different levels comprising incombination: a horizontal load platform, a main frame underlying saidplatform, a plurality of multistage mast assemblies arranged in paralleland interconnecting said main frame and the opposite ends of saidplatform and including an elongated horizontal axis pivot connectionextending transversely of and interconnecting one end of said platformand one of said mast assemblies, separate hydraulic pumps of identicaldisplacement connected in circuit with a respective one of said mastassemblies to supply pressurized fluid thereto and operable in eitherdirection by common prime mover means to extend and retract said mastassemblies in unison, normally closed electrically actuated bleed valvemeans for bleeding fluid from the higher one of said ram assemblies asnecessary to maintain said platform level despite the non-uniformloading of said mast assemblies during both extension and retraction ofsaid platform, and non-level sensor means connected to said platform andresponsive to pivotal movement of said platform about the axis of saidpivotal connection to operate a selected one of said bleed valve meansto bleed fluid from any higher one of said mast assemblies as necessaryto maintain said platform level.
 2. Apparatus as defined in claim 1characterized in that said non-level sensor means is closely adjacentthe upper end of one of said mast assemblies.
 3. Apparatus as defined inclaim 1 characterized in that said non-level sensor means is mounted onthe upper end portion of one of said mast assemblies.
 4. Apparatus asdefined in claim 1 characterized in that said sensor means includes anelectric switch connected in circuit with electrical operating means forsaid bleed valve means and normally so positioned that said bleed valvemeans is closed to block the escape of fluid from all of said mastassemblies so long as said platform is level and operable to open thebleed valve means of the highest one of said mast assemblies when saidplatform is not substantially level.
 5. Apparatus as defined in claim 1characterized in that said main frame is rigidly secured to a chassishaving steerable front wheels and hydraulically power driven rearwheels, and control means for said power driven rear wheels andincluding accumulator means for cushioning initiation and cutoff ofpressurized fluid to said power drive thereby to assure smooth startingand stopping of said chassis.
 6. Apparatus as defined in claim 1characterized in that said prime mover means comprises a reversibleelectric motor.
 7. Apparatus as defined in claim 6 characterized in thatsaid prime mover means comprises a reversible direct current motorenergized from batteries carried by said main frame.
 8. Apparatus asdefined in claim 1 characterized in the provision of hydraulicallypowered chassis means supporting said main frame, and means forselectively operating at any time either said mast assemblies to changethe level thereof or said hydraulically powered chassis means. 9.Apparatus as defined in claim 8 characterized in that said hydraulicallypowered chassis means includes accumulator means in circuit with thepressurized hydraulic fluid powering said chassis means effective toprovide smooth starting and stopping thereof.
 10. Apparatus as definedin claim 1 characterized in the provision of folding protective meanshingedly interconnecting said platform and said main frame andprotectively enclosing portions of the controls for said prime mover andsaid pumps.
 11. Apparatus as defined in claim 10 characterized in thatsaid folding protective means comprises a plurality of elongated rigidmembers hingedly connected together in end to end relation with one endmovably supported at said platform and the other end movably supportedat said main Frame, and flexible control means protected and supportedby said rigid members and interconnecting control means for saidapparatus located on said platform and other portions of said apparatuslocated on said main frame.
 12. Apparatus as defined in claim 1characterized in that said prime mover means comprises unidirectionalpower generating means coupled to said separate hydraulic pumps byreversible power transmission means.
 13. Apparatus as defined in claim12 characterized in that said main frame comprises a hydraulicallypowered steerable chassis, and fluid pump means having a drivingconnection with said unidirectional power generating means in advance ofthe drive connection of the latter to said reversible power transmissionmeans.
 14. Apparatus as defined in claim 12 characterized in theprovision of control means on said platform operable to stop and startsaid power generating means at will from an operating control station onsaid platform irrespective of the elevation thereof above ground level.15. Apparatus as defined in claim 1 characterized in that said primemover means comprises an internal combustion engine coupled to saidseparate hydraulic pumps by reversible power transmission means. 16.Apparatus as defined in claim 15 characterized in the provision ofcontrol means for shifting said power transmission means between aneutral position, forward drive position and reverse drive position froma control station located on said platform.
 17. Apparatus as defined inclaim 16 characterized in that said control means includes hydraulicmotor means located in part on said platform and in part on said mainframe.
 18. Apparatus as defined in claim 1 characterized in that saidmain frame is mounted on steerable front wheels and hydraulically drivenrear wheels.
 19. Apparatus as defined in claim 18 characterized in theprovision of hydraulically powered means for steering said front wheelsfrom a control station on said platform.
 20. A tower assembly as definedin claim 19 characterized in the provision of means for deactivating theoperation of said mast assemblies so long as power is being supplied topropel and/or to steer said chassis.
 21. A tower assembly as defined inclaim 19 characterized in the provision of means for automaticallyrendering the drive means for the pumps connected in circuit with saidmast assemblies ineffective to drive said pumps while said hydraulicallypowered means for said front and rear wheels is activated.
 22. A mobilechassis-supported extendable tower assembly for moving a load betweendifferent levels, said tower assembly comprising a main chassis frameclosely spaced above the ground, a load platform overlying said mainframe including a plurality of inverted upright yoke means overlyingsaid platform and having the lower ends thereof fixed to said platformnear the opposite ends thereof, multistage mast assemblies having theirlower ends rigidly secured to said main frame and their upper endssecured to the upper end of a respective one of said yoke means, saidmast assemblies having a retracted height corresponding generally to theheight of said yoke means, power-driven pressurized fluid means forextending and retracting said masts in synchronism by pressurized fluidsupplied to each of said mast assemblies, said last mentioned meansincluding electrical level sensor means mounted on said platformeffective to sense tilting of said platform away from the normalhorizontal plane thereof and electrically connected to normally closedfluid bleed valves for each of said mast assemblies supportedindependently of said platform and automatically operable to bleed fluidfrom the higher one of said mast assemblies when and as necessary torestore said platform to the level condition thereof during bothextension and retraction of said platform, said assembly being fullyself-contained and operable from a source of power carried by saidchassis.
 23. A tower assembly aS defined in claim 22 characterized inthat said electrical sensor means comprises a pair of electricalswitches spaced from one another in a plane parallel to a plane commonto the axes of said mast assemblies and respectively responsive to aclockwise and counterclockwise tilting of said platform away from thenormal horizontal position thereof to open one or the other of anassociated one of said fluid bleed valves until said platform has beenrestored to the level condition thereof.
 24. A tower assembly as definedin claim 22 characterized in the provision of check valve means locatedin the fluid supply to said mast assemblies and effective to hold saidplatform locked in any elevated position thereof despite a failure inthe operating condition of said power driven means, and manuallyactuated means operable to bleed fluid from each of said mast assembliesto lower the same in synchronism.
 25. A tower assembly as defined inclaim 22 characterized in that said power driven means includes similarreversible positive displacement pumps for each of said mast assembliescoupled to a common prime mover for operation in unison, check valvemeans between a source of hydraulic fluid and the inlet to said pumpmeans to prevent reverse flow of fluid to the fluid source upon theunintended failure of said prime mover, and manually actuated fluidventing means between said check valve means and said pump meansoperable to lower said platform as the fluid in said mast assembliesescapes therefrom via said pump means and thence through said fluidventing means.
 26. A tower assembly as defined in claim 22 characterizedin the provision of manually controlled hydraulic motor means fordriving an axially aligned pair of said chassis wheels and includingaccumulator means for cushioning acceleration and deceleration of saidchassis.
 27. A tower assembly as defined in claim 22 characterized inthe provision of separate reversible hydraulic motors driving arespective one of the chassis wheels to propel said mobile towerassembly forwardly and rearwardly at the operator''s option.
 28. A towerassembly as defined in claim 27 characterized in the provision ofreversible hydraulic means for steering said tower assembly to the rightand to the left from a control station on said platform.
 29. A mobileextendable tower assembly comprising a heavy duty chassis having anelongated rigid main frame, an elongated load supporting platformoverlying said main frame, similar upright tubular housings having theirlower ends fixed to the opposite ends of said platform, a pair ofmulti-stage mast assemblies having their smaller ends secured to theinterior upper ends of said housings and extending downwardlytherethrough to fixed anchorages at the opposite ends of said chassismain frame, a prime mover coupled to separate reversible pump meansoperable to supply pressurized fluid to a respective one of said mastassemblies to raise and lower the same in unison, normally closedbleeder valve means supported independently of said platform for ventingfluid from any higher one of said mast assemblies if said platform tiltsaway from a horizontal plane lengthwise thereof thereby to restore thelevel condition of said platform, and high sensitivity non-level sensormeans including two pairs of electric contactors connected in circuitwith a source of electrical energy and said bleeder valve means andoperable to bleed fluid from any higher one of said mast assembliesduring both extension and retraction of said platform as necessary torestore said platform to a level condition upon sensing a non-levelcondition of said platform.
 30. A tower assembly as defined in claim 29characterized in that said prime mover is coupled to said reversiblepump means by reversible transmission means.
 31. A tower assembly asdefined in claim 29 characterized in the provision of means for loweringsaid platform independently of the operation of said prime mover andunder the control of said non-Level sensor means.
 32. A tower assemblyas defined in claim 29 characterized in that said pairs of contactorsare mounted at the upper end of one of said tubular housings.
 33. Atower assembly as defined in claim 32 characterized in that said primemover means comprises an internal combustion engine.
 34. A towerassembly as defined in claim 33 characterized in the provision ofmanually operable control means on said platform for starting andstopping said engine.